This study is designed to investigate whether exposure to particulate air pollution increases the allergic response to allergens. Research studies suggest that symptoms in individuals with allergies may be aggravated by exposure to particulate air pollution. We sought to experimental determine this by exposing human volunteers to combustion particles, a component of air pollution, and then challenge them with an allergen such as ragweed or oak tree pollen. Using biological tests we can measure whether the allergen response is magnified by prior particulate exposure.